In recent years, there has been used a radio frequency identification (RFID) tag such as a contactless integrated circuit (IC) card that is supplied with a power supply and receives information from an external device such as a reader/writer by using a radio wave in a non-contact manner. The RFID tag includes a transmission/reception antenna pattern formed on a base material such as plastic and an IC chip, and a resonant circuit is formed by the antenna pattern and a capacitive element built in the IC chip. Thus, the RFID tag can wirelessly communicate with the external device through the antenna pattern.
Of such RFID tags, an RFID tag of an ultra high frequency (UHF) band mainly employs a dipole type in which a dipole antenna is used. A dipole type RFID tag is generally attached to, for example, cardboard or clothing other than a metallic material. This is because when the dipole type RFID tag is attached to the metallic material, a gain of the antenna deteriorates, and matching between the antenna and the IC chip collapses, and thus a communication distance is shortened.
Further, there has been suggested a technique of generating a mirror image current (an image current) and forming a current loop by disposing a loop antenna in a direction vertical to a metallic surface. According to such a technique, since a gain of the loop antenna is improved, the communication distance can be increased.
However, in the above-described conventional technique, there is a limitation to increasing the communication distance. Specifically, in the above-described conventional technique, the gain of the antenna can increase by forming a large current loop. However, merely increasing a current loop causes matching between the antenna and the IC chip to collapse. As a result, there arises a problem in that the communication distance is shortened.
Such a problem will be described in detail with reference to FIG. 17. FIG. 17 is a view illustrating the relationship between an IC chip and an antenna through an equivalent circuit. As in an example illustrated in FIG. 17, the relationship between an IC chip and an antenna for a UHF band RFID is expressed by an equivalent circuit. In the example illustrated in FIG. 17, Rcp represents parallel resistance of the IC chip, and Ccp represents parallel capacitance of the IC chip. The parallel resistance Rcp and the parallel capacitance Ccp are fixed values for each IC chip. For example, the parallel resistance Rcp is 2000 [Ω (ohm)], and the parallel capacitance Ccp is 1.0 [pF (picoFarad)].
Further, the antenna and the IC chip are directly connected with each other through two terminals without using a matching circuit therebetween. For this reason, the equivalent circuit at the antenna side is expressed by parallel resistance (radiation resistance) Rap and parallel inductance Lap. In the equivalent circuit illustrated in FIG. 17, when the parallel capacitance Ccp and the parallel inductance Lap satisfy a resonance condition at a frequency f0 of RFID, and the parallel resistance Rcp and the parallel resistance Rap have almost the same value, all power received by the antenna is supplied to the IC chip, and thus communication can be performed. However, when the parallel capacitance Ccp and the parallel inductance Lap do not satisfy the resonance condition at a frequency f0 of RFID or when the parallel resistance Rcp and the parallel resistance Rap have different values, matching between the antenna and the IC chip collapses, and thus the communication distance of the RFID is shortened.
Here, when the current loop increases, the parallel inductance Lap illustrated in FIG. 17 increases. This causes a problem in that matching with the parallel capacitance Ccp of the IC chip that is a fixed value is not made. That is, when the current loop increases, the parallel capacitance Ccp and the parallel inductance Lap may not satisfy the resonance condition, and thus the communication distance of RFID is shortened. Due to the foregoing reasons, in the conventional technique, there is a limitation to increasing the communication distance.    Patent Document: Japanese Patent Application Laid-open No. 2006-53833